This invention concerns improvements in dry inductor-capacitor devices and a method for making the same. Additionally, this invention relates to series impedance circuits that include a voltage level regulating or a power regulating inductor-capacitor unit for gaseous discharge lamps.
A typical dry inductor-capacitor device, also known as a cap-reactor, comprises at least two strips of conducting foil which are separated by a thin layer of dielectric material. These strips are rolled together to form a coil-like unit and electrical terminals are affixed at the start end of one foil and the finish end of the other foil. When used in alternating current regulating circuits, an inductive reactance component is established by parallel current flow through both layers of foil in a direction of the coil. A capacitive reactance component is established by displacement current flow normal to the surfaces of the conducting foils. A magnetically permeable iron core may be inserted in the center of the coil form to increase the inductive reactance component. These types of devices are generally more desirable for providing impedance in power regulating circuits because of their relatively smaller size and weight than comparable impedance devices comprising separate inductor and capacitor elements.
An example of such an inductor-capacitor device and at least one explanation of their theory of operation is described in U.S. Pat. No. 2,521,513 issued to Gray.
In the design and construction of these inductor-capacitor units, the acquistion of good mechanical integrity is desired. Specifically, the manner of affixing connection terminals to the foils has previously presented some difficulties. Usually, an electrical terminal is attached to the start and/or finish end of the foils by cold welding or press fitting techniques. Since the foils are relatively thin, typically less than 0.001 inch, the often encountered bending and pulling of terminal leads during handling and use can cause the terminals to break, or cause the foil to crack near the terminal connection point. Moreover, the cold welding or press fitting of leads at the terminal juncture introduces contact resistance which may be potentially destructive to the unit. The passage of electrical current through the terminal juncture causes heat to be generated thereat which heat may destroy the unit.
Furthermore, good structural integrity also is necessary to counter a magnetomotive force that tends to displace the foils. The magnitude of this magnetomotive force is proportional to and varies with the magnitude of the current flow in the foils. When the foils are not tightly secured in the unit, conventional alternating current sources cause oscillatory movement thereof and eventual degradation of the unit. When the required current handling capability is small, the corresponding magnetomotive forces are small and the typical inductor-capacitor device may be used. However, when relatively large current handling capabilities are demanded, the typical inductor-capacitor unit rapidly degrades. Even with the use of the smaller units, this possible degradation problem has restricted the use of cap-reactors to experimental models only, as no commercially viable unit has yet been developed.
In addition to achieving mechanical and structural integrity, there are yet other difficulties that have been experienced with inductor-capacitor devices. Specifically, a phenomenon known as "current bunching" that results from uneven magnetic fields about the conducting foils will often cause localized "hot spots" to develop in the unit. Discontinuities in the magnetic circuit resulting from gaps in the iron core tend to force the flow of charged particles to one edge of the foil. This phenomenon is not experienced in wire wound inductor coils as current flow is contained in the wires. The consequent overheating resulting from those "hot spots" ultimately cause oxidation of the foil and deterioration of the insulating dielectric. The heat produced by this excessive localized current also destroys the bonding material which holds together the unit. Some of this heat may be dissipated through radiation from an iron core. Thus, the upper limit of the current handling capability of an inductor-capacitor device, as limited by the effects of "current bunching", is established by the heat transfer characteristics of the inductor-capacitor device. The heat transfer capability is greater in dry inductor-capacitor devices than the liquid dielectric type, as the liquid dialectric, such as oil, will contain the heat in the fluid medium.
At least one solution to the difficulties that result from the "current bunching" phenomenon is to more uniformly distribute the current through the conducting foil. An example of one current distribution system is shown in U.S. Pat. No. 3,688,232 is issued to Szatmari. In one example shown therein, the conducting foils vary in thickness according to their position in the coil. The starting portion of the foil that receives current is thicker, and the finish portion of the same foil is relatively thinner. Conversely, the finish portion of the second conducting foil is thicker and the start portion of that foil is relatively thin. The thicker foil portions have greater cross-sectional areas for greater current handling capabilities. Another device, disclosed in that same patent, discloses branching foil elements for handling increased current flow at certain positions in the coil unit. Obvious disadvantages of these structures are the complex assembly and fabrication requirements which may not render the device cost effective.
The minimization of leakage current that occurs between the conducting plates through the dielectric material is yet another design consideration. Leakage current is defined as a current that flows through or across the surface of insulating dielectric material and defines the insulation resistance at the specified voltage potential. In conductor-capacitor devices, increased leakage current will result in increased leakage capacitance. A proper selection of dielectric material, together with the proper selection of bonding material, can improve the optimum performance characteristics of inductor-capacitor devices. Prior art teaching, to the inventor's knowledge, to this problem has not been addressed.
Respecting circuit applications, a variety of inductor-capacitor devices have been incorporated in a variety of alternating current power regulating circuits, such as gaseous discharge lamp circuits. A unitary inductor-capacitor device, instead of separate inductive and capacitive elements, is particularly suitable in discharge lamp circuits because of their relative reduced size, weight and cost. Circuit design objectives include power factor correction, open circuit starting voltage, operating voltage and current limitation.
The open circuit starting voltage is provided by the ratio of turns between the primary winding to which the power supply is connected and the secondary winding to which the lamp device is connected. Power factor correction, a function of capacitance, is determined by the effective plate area (number of turns of foil) of the conducting foil strips and the dielectric constant of the insulating material placed therebetween. The operating voltage and current is provided by inductive reactance that is determined by the number of turns of conducting foil in series with the lamp and the magnetic reactance provided by the magnetic core material, if inserted. Thus, it can be seen that the capacitance, inductance, start-up voltage, and operating voltage characteristics of the circuit are all interplayed. A change or alteration, of one parameter may necessarily effect the other parameter. Accordingly, one may propose a variety of combinations of the several parameters in the design of regulating circuits for discharge lamp devices.
Once having established the design parameters of an inductor-capacitor device that possesses the required current handling capabilities, further innovation is necessary to produce an electrical impedance circuit having desired performance characteristics. It is desirable to choose an inductor-capacitor circuit that allows flexibility in selecting the physical parameters so that one may conveniently meet power factor correction, open circuit voltage, operating voltage, and current limitation.
In view of the foregoing, it is an object of this invention to provide an improved inductor-capacitor device and a method for making the same.
Another object of this invention is to provide an improved lamp ballast power regulating circuit incorporating an improved inductor-capacitor device.
Another object of this invention is to provide an inductor-capacitor device having terminals that are more durable than prior art units by integrally forming terminals on the conducting foil.
Another object of this invention is to provide a dry inductor-capacitor device having a greater mechanical integrity than prior art devices.
Another object of this invention is to provide an inductor-capacitor device that possesses improved heat transfer characteristics and current distribution through the conducting foils.
A further object of this invention is to provide an inductor-capacitor device having a higher power handling capability than prior art devices.
It is yet another object of this invention to provide an inductor-capacitor device in a power regulating circuit for a discharge lamp wherein greater flexibility in selecting physical parameters of the device are afforded.
Further, an additional object of the invention will become more readily apparent upon review of the succeeding disclosure taken in connection with the accompanying drawings.